Some oil and gas wells produce sand and silt along with produced liquid. Progressing cavity pumps (PCPs) are particularly suited to these applications because they can mobilize such fluid mixtures without sustaining significant damage. Although PCPs can typically move dirty fluid through the pump, the solid material must still be conveyed all the way to the surface, which in some cases is a distance of 1000 meters or more. It has long been understood that the solid particles will be carried to the surface by the liquid as long as the upward velocity of the surrounding liquid exceeds a particular critical velocity. If the liquid velocity falls below the critical velocity, solid particles will begin to accumulate. If enough material settles in the production tubing, a flow restriction is created. This restriction results in increased torque requirement for the pump and can further decrease fluid velocity. If the solid material accumulates at the top of the pump, it can interfere with the rotation of an eccentric rotor pin and sucker rod drive string. The problem can gradually compound, resulting in a situation where the pump can no longer pump fluid to the surface. A well in this final state is commonly termed “sanded up” or “silted up”.
Traditionally, oil and gas production operators often allow wells to pump until an unusually high torque load is noted. In some cases, the situation goes unnoticed and the well may “sand up.” In either case, an intervention is required. The intervention might involve manually dumping liquid down the casing/tubing annulus and trying to pump at a higher speed in an attempt to create flow in the regime of critical velocity. This procedure sometimes alleviates the problem, but, quite commonly, a more elaborate intervention is needed. The enhanced intervention involves lifting a sucker rod string coupled to the PC pump to disengage the PCP rotor from the stator. Because PC pumps are rarely installed with a standing valve, this opens an unrestricted conduit within the tubing from the surface down to the pump intake. Fluid can then be pumped down the tubing to reverse the flow of remaining solid material down the tubing and out into the casing annulus. The rotor can then be re-seated and pumping resumed.
Such interventions, however, are costly and result in considerable unproductive time for the well. In some cases, the action taken to clear the well conduit can result in sand or silt being pushed into the producing formation, which is an undesirable situation that might detract from the ultimate recovery of the well.
Considerations for Coalbed Methane Wells
Although solids accumulation in the production conduit is an undesirable situation for any oil or gas well, it is especially problematic in coalbed methane wells. In coalbed methane production, the downtime associated with a “sand up” condition is particularly detrimental to production. When the well is placed back in pumping operation it may take days before any gas production resumes. Coalbed methane wells are also more difficult to keep clean because the water produced has a lower viscosity than the oil produced by most oil wells. Because the critical velocity required to carry solids to the surface is inversely proportional to liquid viscosity, significantly higher flow rates need to be maintained in coalbed methane wells to sustain a clean production conduit. The situation for coalbed methane wells is particularly compounded by the fact that water production from these wells declines as the well continues to be pumped. In general, the water production rate from these wells continuously declines while the gas production rate increases. As a result, the producing formation is often providing inadequate quantities of water to sustain critical velocity through the pump and tubing.
One approach to alleviating this problem is to continuously inject supplemental quantities of additional clean liquid at or near the pump intake. With the reservoir liquid volume supplemented by this injected volume, the pump could be run at a higher rate. Using this technique, critical velocity could be continuously maintained within the tubing. For this type of operation, the liquid could simply be dumped down the casing annulus and allowed to fall to the pump intake. Such an approach is sometimes utilized. But, again, coalbed methane wells pose a particular problem for this technique. Since the casing annulus is used as a production conduit for the gas phase, it is difficult to get the injected liquid (water) to fall against the gas flow in the annulus.
In some installations a third conduit (in addition to tubing and casing-tubing annulus) is installed in the well to facilitate continuous liquid injection. Hollow sucker rods are one means of providing an additional conduit, but add significant cost and complexity to the installation.
A modification of the continuous dump procedure involves intermittent introduction of a slug of liquid into the wellbore while simultaneously increasing the pump rate. Such a procedure could be used to wash the accumulated solids from the tubing. In order to inject the slug of liquid, the casing flow valve would need to be temporarily closed to allow the liquid to fall to the bottom of the well. If the procedure is executed at the proper frequency, the tubing could be kept relatively clean of solids accumulation. The procedure could be executed manually using a water tank truck with pump and a human operator to perform all steps in the procedure. Because of the time required to complete the procedure on a single well, it is likely that one operator could only wash four or five wells per day. In a small operation, this manual operating approach might be acceptable.